The present invention relates to polarizing laminations, of the type which may be used as the polarizing elements in optical systems, such as liquid crystal displays.
Polyvinyl alcohol (PVA) has been commonly used as a polarizing film. Polyvinyl alcohol when stretched and treated with a halide provides a film with polarizing qualities. Polyvinyl alcohol is water soluble and thin films of PVA are structurally weak. Accordingly, prior art workers in the field found it necessary to laminate the PVA film to a structurally strong substrate as it was being processed. Also, some crystalline materials were found to produce undesired optical effects including colorful rainbow and dispersive effects. Accordingly, during the processing of the PVA film supporting films of noncrystalline material were used. Initial laminates such as glass and cellulosic materials were widely used. These laminates tended to be bulky and made application and use of the polarizing laminations difficult.
The modern trend in the field of polarizing laminations has been toward the development of a film which is as thin as possible while still protecting the polyvinyl alcohol from moisture and providing an adequate structural support. Cellulose acetate butyrate is a widely used material for laminating the polyvinyl alcohol. However, problems have been encountered in that the cellulose acetate butyrate/PVA laminate is moisture sensitive and tends to curl and separate from the polyvinyl alcohol along the edges of the film in the presence of heat and/or humidity drawing air into the pressure sensitive adhesive between the two layers. Moisture then attacks the water soluble polyvinyl alcohol along the edges of the film, which results in a polarizing lamination of inferior optical qualities, and dissatisfaction on the part of the owner of equipment including the film. Additionally, moisture attacking the cellulose acetate butyrate causes bubbling and curling at the interface with the polarizing material or other substrate.
Acrylic type films have recently been used with some success; however, thin acrylic films tend to be brittle and crack easily. They also tend to curl when exposed to high humidity. The cracking also results in moisture attack on the polyvinyl alcohol having the undesired results noted above.
It is therefore an object of the present invention to disclose and provide a laminating material with sufficient strength when used in thin films to provide support and protection for polyvinyl alcohol films.
Another object of the present invention is to disclose and provide a polarizing lamination film which is not brittle and does not exhibit peeling characteristics around the edges of the lamination.
A further object of the present invention is to disclose and provide an economical method for applying an improved lamination material to polyvinyl alcohol.
An additional objective of the present invention is to disclose and provide an ultrathin polarizing lamination which is particularly useful in liquid-crystal display applications.
The proposed lamination material should be structurally strong even in thin layers as thin as two thousandths of an inch while protecting the polyvinyl alcohol from moisture and providing the required structural support. The new lamination material must not interfere with the optical qualities of the system utilizing the polarizing layer.